This application is related to Japanese Patent Application No. 2000-296713 filed on Sep. 28, 2000, whose priority is claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a surface acoustic wave resonator and a surface acoustic wave filter using the same, particularly a ladder type filter.
2. Description of the Related Art
A surface acoustic wave filter and a resonance circuit using a surface acoustic wave resonator can be provided with a compact size and a low cost. Therefore, a surface acoustic wave resonator is one of the necessary constitutional elements for reducing the size of recent communication equipments, such as a portable phone.
FIG. 17 is a constitutional diagram showing a conventional ordinary surface acoustic wave resonator.
The surface acoustic wave resonator comprises a piezoelectric substrate 1 having thereon a interdigital transducer (IDT) 2 formed with an aluminum alloy having a period corresponding to a desired frequency, and reflectors 3-1 and 3-2 reflecting a surface acoustic wave excited by the interdigital transducer 2. The electrode period pi of the interdigital transducer 2 can be obtained from the velocity vi of the surface acoustic wave on the substrate at the interdigital transducer and the desired frequency fi by the following equation:
pi=vi/fi
The surface acoustic wave resonator shown in FIG. 17 is a single terminal pair resonator, in which one of the end parts of the interdigital transducer 2 is an input electrode 2-1, to which an input signal is applied, and the other thereof is an output electrode 2-2, from which an output signal is taken out. The reflectors 3-1 and 3-2 are generally formed with a grating having periodicity.
While the grating can be formed by making grooves on the piezoelectric substrate, an aluminum alloy grating is generally used, which can be formed simultaneously with the interdigital transducer.
The grating period pr can be obtained, as similar to the case of the interdigital transducer, from the velocity vr of the surface acoustic wave at the reflector and the desired frequency fr by the following equation:
2xc3x97pr=vr/fr
In general, as fi=fr, assuming that vi and vr are substantially the same as each other, the design is often made with pi=2xc3x97pr.
Herein, twice the grating period pr is sometimes referred to as a period of the reflector. In this case, the reflector is sometimes referred to as xe2x80x9ca half-period reflectorxe2x80x9d.
In general, the interdigital transducer 2 has been formed with a single electrode comprising two electrode fingers within the electrode period pi. The reflector has also been generally formed with a single electrode as similar to the interdigital transducer 2 since two grating electrode fingers 3-3 are present within twice the grating period pr, which is the same as the electrode period pi.
The single electrode herein has such a constitution that the electrode fingers of the interdigital transducer are arranged where one electrode finger extending from the end part of the input electrode 2-1 and one electrode finger extending from the end part of the output electrode 2-2 are alternately arranged. That is, one electrode finger extending from the end part of the output electrode 2-2 is necessarily arranged between two adjacent electrode fingers extending from the end part of the input electrode 2-1.
The electrode fingers thus alternately arranged each are referred to as a single electrode finger.
FIG. 18 is a constitutional diagram showing a conventional double terminal pairs resonator comprising plural interdigital transducers, in which numerals 2-3 and 2-4 denote ground terminals.
FIG. 19 is a diagram showing the simplest electrically equivalent circuit of a single terminal pair surface acoustic wave resonator formed on a piezoelectric substrate 1, such as quartz and LiTaO3. A single terminal pair surface acoustic wave resonator is used by electrically connected in serial or in parallel as shown in FIGS. 20(a) and 20(b) or FIGS. 21(a) and 21(b).
In FIG. 19, symbol R1 denotes a resistance, C0 and C1 denote capacitance, Li denotes an inductance, Ti denotes a terminal of the input electrode 2-1, and To denotes a terminal of the output electrode 2-2.
Herein, R1, C1 and L1 are such values that are determined by the material of the piezoelectric substrate, and C0 is a value varying depending on the number of pairs of the interdigital transducers.
In the case of the serial connection shown in FIG. 20(a), a single terminal pair surface acoustic wave resonator R is arranged in serial between the input Ti and the output To as shown in FIG. 20(b). In the case of the parallel connection shown in FIG. 21(a), a single terminal pair surface acoustic wave resonator R is arranged in parallel between the pair of the input Ti and the output To, and the ground G as shown in FIG. 21(b).
FIG. 22 is a diagram showing general frequency characteristics in the case where the single terminal pair surface acoustic wave resonator is connected in serial. Herein, the abscissa indicates the frequency (Hz), and the ordinate indicates the attenuation amount (dB). According to the diagram, the attenuation amount exhibits the maximum value at a certain frequency, which is referred to as an antiresonance frequency fas.
FIG. 23 is a diagram showing impedance characteristics in the case where the single terminal pair surface acoustic wave resonator is connected in serial. Herein, the abscissa indicates the frequency, and the ordinate indicates the absolute value of impedance (logarithmic value). According to the diagram, double resonance characteristics are observed, in which a resonance frequency frs, at which the impedance shows the minimum, appears on the low frequency side, and an antiresonance frequency fas, at which the impedance shows the maximum, appears on the high frequency side.
FIG. 24 is a diagram obtained by overlapping FIG. 22 and FIG. 23. In this figure, a part to be a pass band of the ladder type filter and a part to be an attenuation band of the ladder type filter are shown.
FIG. 25 is a diagram showing general frequency characteristics in the case where the single terminal pair surface acoustic wave resonator is connected in parallel, and FIG. 26 is a diagram showing impedance characteristics in the case where the single terminal pair surface acoustic wave resonator is connected in parallel. Herein, the ordinate of FIG. 25 indicates the absolute value of admittance (logarithmic value).
In these figures, the frequency, at which the attenuation amount becomes minimum, is the resonance frequency frp, the frequency, at which the admittance becomes maximum, is the resonance frequency frp, and the frequency, at which the admittance becomes minimum, is the antiresonance frequency fap. In the case of the parallel connection, the double resonance characteristics having two resonance frequencies frp and fap are exhibited.
The surface acoustic wave resonator of this type is used singly or as a combination of plurality thereof as a ladder type filter. FIG. 27 is a constitutional diagram showing an example of the ladder type filter. In the ladder type filter as shown in FIG. 27, several surface acoustic wave resonators (S1, S2, R1 and R2) are connected in parallel and serial. At this time, the interdigital transducers of the respective resonators are designed in such a manner that the antiresonance frequency fap of the parallel resonators R1 and R2 substantially agree with the resonance frequency frs of the serial resonators S1 and S2.
FIG. 28 is a diagram showing general frequency characteristics of a ladder type filter. The ladder type filter is a band pass filter passing a certain frequency band.
Characteristic values demanded in a band pass filter include the pass band width BW1 which is shown in FIG. 28, the attenuation band widths BWatt1 and BWatt2, and the attenuation degrees of the attenuation bands ATT1 and ATT2.
The ratio (BW1/BW2) of the band widths BW1 and BW2 at a certain attenuation amount is referred to as a shape factor, which is used as a characteristic value in the case where steep characteristics of the band are required. In general, the better, the closer the shape factor is to 1, which provides a filter of high squareness.
Representing the center frequency of the pass band of the filter by f0, the values obtained by normalizing BW1 and BW2 with the center frequency f0 (BW1/f0 and BW2/f0) are referred to as fractional band witdths.
As shown in the characteristic diagram of the serial resonator in FIG. 24, the right hand part of the antiresonance frequency fas is a part to be an attenuation band of the ladder type filter, which corresponds to the part of BWatt2 in FIG. 28.
The part of flat pass characteristics on the left hand of the antiresonance frequency frs in the vicinity of the antiresonance frequency frs in FIG. 24 is a part to be a pass band of the ladder type filter, which corresponds to the part BW1 in FIG. 28.
In the ladder type filter, as understood from FIG. 24 and FIGS. 22 and 25, the pass band widths BW1 and BW2 are substantially determined by the distance between the antiresonance frequency fas of the serial resonator and the resonance frequency frp of the parallel resonator.
The resonance frequency frs and the antiresonance frequency fas of the surface acoustic wave resonator are substantially determined by the material of the piezoelectric substrate 1. In particular, the band width of the ladder type filter is substantially determined by the electromechanical coupling coefficient among the characteristics of the piezoelectric substrate material.
For example, while a frequency of f0=836.5 MHz is used in AMPS (Advanced Mobile Phone Service) in the U.S., the characteristic values of the specification thereof demand a pass band width of 25 MHz and a fractional band witdth of about 3%. Such a band pass filter of wide band has been realized only by a piezoelectric substrate of a high electromechanical coupling coefficient, such as 36xc2x0 Y-cut X-propagation LiTaO3.
When a piezoelectric substrate having a large electromechanical coupling coefficient is used, a band pass filter of wide band can be obtained since the resonance and antiresonance frequencies of the surface acoustic wave resonator are apart from each other. However, the shape factor is deteriorated at the same time. That is, there is a tendency that the shape factor becomes small when the fractional band witdth is increased.
The resonance and antiresonance frequencies are substantially determined by the piezoelectric substrate since the electromechanical coupling coefficient is a value inherent in the substance, and they cannot be arbitrarily obtained. Therefore, it has been difficult to adjust the fractional band witdth and the shape factor to the desired values.
The present invention relates to a surface acoustic wave resonator comprising a piezoelectric substrate, a interdigital transducer part which is formed on a piezoelectric substrate and is composed of plural electrode fingers having a period pi that is substantially equal to a wavelength of a surface acoustic wave to be excited, and at least one reflector arranged in the vicinity of the interdigital transducer part to reflect the surface acoustic wave excited by the interdigital transducer part in a direction parallel to a propagation direction of the surface acoustic wave, wherein the interdigital transducer part has three or more electrode fingers within the period pi, and the reflector is composed of plural gratings having a period pr that is equal to a half of a wavelength of a surface acoustic wave propagating in the reflector.
According to the present invention having the specific structure of the interdigital transducer of the surface acoustic wave resonator, the distance between the resonance frequency and the antiresonance frequency of the surface acoustic wave resonator is decreased even though the same piezoelectric substrate material as the conventional art is employed, and in the case of a ladder type filter, a filter of higher squareness can be realized with the demanded fractional band width.